virtio: pci: check bar values read from virtio config space

virtio pci config structures may in future have non-standard bar
values in the bar field. We should anticipate this by skipping any
structures containing such a reserved value.

The bar value should never change: check for harmful modified values
we re-read it from the config space in vp_modern_map_capability().

Also clean up an existing check to consistently use PCI_STD_NUM_BARS.
Signed-off-by: Keir Fraser <keirf@google.com>
Link: https://lore.kernel.org/r/20220323140727.3499235-1-keirf@google.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_pci_modern.c

@@ -293,7 +293,7 @@ static int virtio_pci_find_shm_cap(struct pci_dev *dev, u8 required_id,
 
 	for (pos = pci_find_capability(dev, PCI_CAP_ID_VNDR); pos > 0;
 	     pos = pci_find_next_capability(dev, pos, PCI_CAP_ID_VNDR)) {
-		u8 type, cap_len, id;
+		u8 type, cap_len, id, res_bar;
 		u32 tmp32;
 		u64 res_offset, res_length;
 
@@ -315,9 +315,14 @@ static int virtio_pci_find_shm_cap(struct pci_dev *dev, u8 required_id,
 		if (id != required_id)
 			continue;
 
-		/* Type, and ID match, looks good */
 		pci_read_config_byte(dev, pos + offsetof(struct virtio_pci_cap,
-							 bar), bar);
+							 bar), &res_bar);
+		if (res_bar >= PCI_STD_NUM_BARS)
+			continue;
+
+		/* Type and ID match, and the BAR value isn't reserved.
+		 * Looks good.
+		 */
 
 		/* Read the lower 32bit of length and offset */
 		pci_read_config_dword(dev, pos + offsetof(struct virtio_pci_cap,
@@ -337,6 +342,7 @@ static int virtio_pci_find_shm_cap(struct pci_dev *dev, u8 required_id,
 						     length_hi), &tmp32);
 		res_length |= ((u64)tmp32) << 32;
 
+		*bar = res_bar;
 		*offset = res_offset;
 		*len = res_length;
 

drivers/virtio/virtio_pci_modern_dev.c

@@ -35,6 +35,13 @@ vp_modern_map_capability(struct virtio_pci_modern_device *mdev, int off,
 	pci_read_config_dword(dev, off + offsetof(struct virtio_pci_cap, length),
 			      &length);
 
+	/* Check if the BAR may have changed since we requested the region. */
+	if (bar >= PCI_STD_NUM_BARS || !(mdev->modern_bars & (1 << bar))) {
+		dev_err(&dev->dev,
+			"virtio_pci: bar unexpectedly changed to %u\n", bar);
+		return NULL;
+	}
+
 	if (length <= start) {
 		dev_err(&dev->dev,
 			"virtio_pci: bad capability len %u (>%u expected)\n",
@@ -120,7 +127,7 @@ static inline int virtio_pci_find_capability(struct pci_dev *dev, u8 cfg_type,
 				     &bar);
 
 		/* Ignore structures with reserved BAR values */
-		if (bar > 0x5)
+		if (bar >= PCI_STD_NUM_BARS)
 			continue;
 
 		if (type == cfg_type) {